The invention is directed to a thermal fixing system for fixing toner images on the front side of a web-shaped recording medium in an electrographic printer or copier device, whereby the back side of the recording medium can already have a fixed toner image.
Thermal fixing devices that comprise a pre-heating saddle with a following fixing zone composed of a heated fixing drum and a pressure roller are employed in printer or copier devices for heat transfer fixing of toner images on a recording medium that is usually composed of paper.
Such thermal fixing devices are disclosed, for example, by U.S. Pat. No. 4,147,922 or Japan Abstract Vol. 13, No. 120, Mar. 24, 1989 (Japan-A-63-292177).
It is beneficial in electrographic printer devices that work in the highest speed range with, for example, a printing speed of more than 0.5 m/s, and that employ a heat transfer fixing station for fixing, to heat the paper web or the paper sheet to temperatures of approximately 100xc2x0 C. or more before the actual heat transfer fixing process in order to thus obtain a good joining of the toner image to the paper surface.
When a paper web or a single sheet of paper that is already printed and fixed on one side, for example on the back side, is to be printed and fixed on the other side, then the first side which is already fixed must be conducted over the hot surface of the pre-heating saddle for heating the paper for the second fixing process. The following problems thereby arise in this second fixing process:
a) Continuous printer operation:
The print image that is already fixed and that runs over the hot surface of the pre-heating saddle is heated to such an extent that it assumes a condition ranging from tacky through fluid, and is partly smeared on the saddle surface. The more toner is transferred from the toner image onto the saddle surface the more toner collects on the saddle surface, until a visible destruction of the toner image on the paper occurs.
b) Waiting or Standby Operation:
While the printer is in the waiting or standby mode, the paper web having the already fixed print image lies on the hot saddle. The print image is heated to such an extent in the region of the surface of the pre-heating saddle that it assumes a tacky through fluid condition and sticks to the hot surface of the pre-heating saddle. When the paper web is started, the toner image is then torn from the surface of the paper web and remains sticking on the hot surface of the saddle.
In the case of the known fixing devices, there is another problem. It has previously been assumed that it is necessary to preheat the paper very rapidly over a relatively short path, via the preheating saddle, and then to fix the toner image on the paper via the rollers. For this purpose, the heating elements are arranged in the preheating saddle in such a way that the greatest quantity of heat is emitted to the recording substrate in the region of the paper inlet of the preheating saddle and that the emitted quantity of heat is then reduced over the heating elements in the direction of the paper exit. Thus, the relatively hottest region of the saddle is the paper inlet.
However, it has appeared that a rapid heating up of the paper over a short path leads to a high loading of the paper. This loading is expressed as a deformation, an embrittlement or an ageing of the paper and as a non-uniform loss of water from the paper during passage through the fixing station. Hence, post-processing of the paper by cutting or sorting is made more difficult or there occurs a non-uniform fixing of the toner images and thus an impairment of the quality of the print.
In addition, a rapid heating up requires a high specific heating power using high-power heating elements and a complicated control system. Because of the high heating power it is therefore necessary to lift the recording substrate immediately from the saddle in the event of a printer stop, in order to prevent buning of the paper. His makes comprehensive control devices necessary, which impairs the paper handling as a whole.
In modem electrophotograhic printing devices, furthermore, recording substrates of the most different widths are processed in the same machine. If the same amount of energy is fed to the saddle over the entire width, the saddle heats up severely in that region where there is no paper running, since in this region no energy is dissipated, apart from losses due to convection.
A temperature distribution of this type has considerable disadvantages. The paper is heated up non-uniformly, which leads to fluctuations in the fixing quality and can also cause paper running problems. The maximum heating saddle temperature must be reduced, since there exists the risk of overheating of the heating elements and the lifetime of the heating elements is thereby shortened. The energy losses are relatively large and the inner region of the machine is heated up unnecessarily.
In the case of thermofixing devices with a preheating saddle, the recording substrate is guided over a heated gliding surface of the saddle. Direct contact between paper and saddle is essential for a good thermal transfer between paper and saddle surface. In the case of high printing speeds and in the use of pre-folded papers or papers of non-uniform thickness, fluttering movements of the paper can occur in the region of the saddle. In consequence, the paper lifts partially off from the saddle, which impairs the thermal transfer. Also, paper contains a relatively high proportion of water, which is released during warming. The released steam can be deposited in the machine and can lead there to disturbances or to corrosion.
It is an object of the invention to provide a thermal fixing means having a pre-heating saddle for fixing toner images on the front side of a recording medium in an electrographic printer or copier device, whereby the back side of the recording medium can already have a fixed toner image.
It is another object of the invention to provide a thermofixing device and a process for fixing, in which the recording substrate is exposed to as small a thermal loading as possible during passage through the fixing device.
It is a further object of the invention to provide a thermofixing device which makes it possible, without fluctuations of the fixing quality, to fix recording substrates of the most different widths and in which warping and deformations of the fixed recording substrate are avoided.
According to the invention, a thermal fixing system is provided for fixing toner images on a front side of a recording medium in an electrographic printer or copier device wherein a back side of the recording medium already has a fixed toner image. A heat transfer fixing station is provided for fixing the toner images on the recording medium. A pre-heating saddle precedes the heat transfer fixing station and a running direction of the recording medium has a sliding surface allocated thereto for accepting the recording medium over its back side. The sliding surface comprises a toner-repellant material at least in a contact region of the recording medium.
The specification of front side and back side of a recording medium is a purely relative matter for describing the two sides of a recording medium.
When the recording medium, which can be composed of single sheets or of continuous form paper, is conducted over a pre-heating saddle having a sliding surface that exhibits a repellant property for the tacky through fluid toner and has high abrasion resistance with respect to the paper web sliding thereon, then the thermal fixing means can be employed in printer or copier devices that work both in a simplex as well as in a duplex mode.
Materials that are manufactured of fluorine compounds such as, for example, PTFE or, respectively, PFA compounds, have proven beneficial. The material can be vapor-deposited, sprayed, or glued on an appropriate acceptance surface of a pre-heating saddle. PTFE or, respectively, PFA compounds exhibit extremely good repellency with respect to the toner material and exhibit extremely good properties regarding abrasion, due to the paper web.
In order to enhance the abrasion resistance, wear-reducing constituents such as graphite or glass fibers can be mixed to the PTFE or PFA to a more or less pronounced degree.
Since such pre-heating saddles are usually utilized in electrographic printer devices of the higher performance category (between 2 and 10 million DIN A4 pages per month), non-wearing operation over years is impossible. For this reason, it is meaningful when the saddle surface can be unproblematically and simply renewed as needed, without the expensive base structure of the heating saddle with heating elements having to be renewed. For this purpose, a toner-repellant layer can be vapor-deposited, sprayed, or glued onto thin metal plates, whereby these coated, individual plates are then interchangeably secured on the base structure of the pre-heating saddle.
In an advantageous embodiment of the invention, the toner-repellant layer is executed as a film which has a thin, thermally conductive adhesive layer on one side. The adhesive layer is implemented such that the film can be easily pulled from the saddle in the hot condition of the saddle. A fast renewal of the saddle surface is thus rapidly possible, as needed on site by the customer.
The toner-repellant layer can also be implemented as a thin film that is taken from a supply reel, is guided over the surface of the pre-heating saddle and is then again wound up. The film is thus moved extremely slowly relative to the running direction of the paper.
In order to obtain a fold-free entry of the paper web into the fixing gap between fixing drum and pressure drum, it has already been proposed to design that end of the pre-heating saddle facing toward the fixing gap as a smoothing edge over which the recording medium is deflected to a great degree. However, extremely high wear of the toner coating on the recording medium occurs in the wrap region in the region of the smoothing edge. This wear can be prevented when rollers that may potentially be provided with a toner-repellant coating are provided in the wrap region.
When a relatively high proportion of graphite or glass fibers is added to the toner-repellant material in order to achieve high wear resistance of the surface, then the repellency of the surface relative to the toner image may potentially be reduced. In order to prevent a transfer of the toner image onto the saddle surface in such cases during a long waiting or standby mode of the printer devices, it is beneficial to lift the recording medium off from the saddle surface. This can occur wherein an air pillow is produced between the paper web and the saddle surface or sliding surface with the assistance of a blower means in the standby condition of the printer device. Another possibility for lift-off is comprised in providing a suitable lift-up element designed, for example, as a tension wire that engages under the recording medium over its entire width. The pre-heating saddle and lift-off element are thereby moved relative to one another such that, in a lift-off status, the recording medium is guided over the lift-off element at a distance from the pre-heating saddle.
As a rule, the paper web is automatically placed into the printer in electrographic continuous form printers of the new generation. Among other things, the paper web must thereby be guided over the pre-heating saddle. Coatings composed of fluorine compounds electrostatically charge at their surface when paper slides thereon. Due to the electrostatic forces, the paper web adheres so firmly to the pre-heating saddle that it may potentially no longer be capable of being transported. An advantageous admixture of electrostatically conductive substances such as graphite or the like can prevent the formation of electrostatic charges. It is beneficial, given glued layers of material, when the adhesive is likewise conductive in order to thus produce a conductive connection between toner-repellant material and grounded carrier.
Also, according to the invention, if the saddle is configured as a low temperature saddle with as large a constructional length as possible, so that the temperature difference between recording substrate and saddle becomes as small as possible, and if, furthermore, the saddle is subdivided in the recording substrate running direction into heating zones which are individually controllable and uniformly heated, the heating zones can then be controlled in such a way that, along the saddle, an approximately constant thermal energy flow occurs from the saddle to the recording substrate.
By means of this measure, the thermal loading for the recording substrate becomes very low. Nevertheless, the thermofixing device can also be used in printing devices of high and very high printing speed.
Furthermore, the subdivision of the saddle, transversely to the recording substrate running direction, into heating zones which can be driven as a function of the width of the recording substrate is of advantage.
In consequence, the heating behavior of the saddle can be matched directly to the width of the recording substrate running through, which guarantees a constant fixing quality, irrespective of the width of the recording substrate used.
In order to make possible a good contact between recording substrate and gliding surface of the saddle, irrespective of printing speed and paper used, in an advantageous embodiment of the invention openings can be arranged on the gliding surface, said openings being connected to a device producing a vacuum. By means of the vacuum, the recording substrate is sucked flat onto the gliding surface and, in the process, the steam released in the paper is simultaneously sucked away via the openings.
Furthermore, if use is made for heating elements of heating cartridges which are arranged in passage openings of the heating saddle, said heating cartridges can easily be exchanged and the saddle itself can be cost-effectively produced from an extruded profile.
A domed shaping of the gliding surface of the saddle ensures a force component, which pressed the recording substrate against the saddle surface, over the entire saddle length. This measure supports the contact of the recording substrate on the saddle surface, stabilizes the recording substrate guidance and thus leads to an improved thermal transfer.
In a further advantageous embodiment of the thermofixing device, peripheral entry means, for example in the form of a keyboard, are provided on the machine, via which means, by means of the entry of operating parameters such as paper weight, fixing temperature, etc., the heating power of the fixing device is automatically matched to these parameters.
Embodiments of the invention are shown in the drawings and shall be set forth in greater detail below by way of example.